How many individual virus are needed to start an infection?

[antares4141]

sometimes Quora has some pretty good answers to questions like these.

How many individual virus are needed to start an infection? Will 1 flu virus be enough to make you sick or do you need many more than that?
https://www.quora.com/How-many-indi...e-you-sick-or-do-you-need-many-more-than-that

is the severity of cold and flu symptoms related to initial virus load? If it takes only one virus to infect us, why do we bother with sanitizers and masks?

https://www.quora.com/Is-the-severi...us-why-do-we-bother-with-sanitizers-and-masks

 

[andyguitar]

Among other things it will depend how good the virus is at getting through the mucus barrier in the respiratory tract.

 

[Wishful]

Thanks. I wondered about that.

 

[ljimbo423]

is the severity of cold and flu symptoms related to initial virus load?

This is from the article-

Many viral (and bacterial) infection severity and incubation time depend upon the size of the viral exposure/inoculum. Chickenpox is (well, was) a classic example, where siblings of the index case (the kid who brought it home from school) were typically sicker than the first kid, presumably from receiving a larger dose of viruses.

I had never heard of this until the other night. I was listening to a virologist and she said the severity of a covid infection often depends on how big the initial exposure to the virus was.

I always thought it didn't matter, as long as you were exposed to enough of the viruses to make you sick but I guess that's not right at all. I find this really interesting!

 

[andyguitar]

I had never heard of this until the other night. I was listening to a virologist and she said the severity of a covid infection often depends on how big the initial exposure to the virus was.

This was something that was mentioned fairly early on over here. Given by one uk expert as the reason some medical staff were much more at risk. Particularly those who were intubating patients for ventilation. Judging by the fear many here have that they can get infected simply by walking past an infected person it seems I was one of the few people who saw the interview. Scince then it's not been mentioned much, if at all.

 

[Hufsamor]

Ive been wondering a lot about those very sick medical staff.
I figured it would have to do with how big the exposure was, but I couldn't find the explanation anywhere. I have tried to google several times.

Thanks !

 

[andyguitar]

Ive been wondering a lot about those very sick medical staff.

As the risk of infection for medical staff is high so to is the risk they will infect others. Including any patients they have contact with. I suspect this is the main route of infection. But nothing gets said about it.

 

[antares4141]

(out of quora) Some viruses replicate slowly enough that unless there is a goodly number of infected cells to start with, your immune or interferon system will kick its butt before you feel the first inkling you are sick.

So if this happens to you you will test positive for antibodies meaning you were one of the lucky "asymptomatic" Which implies you were just better able to fight it off.

When the reality probably is more like your very limited exposure was the equivalent of a vaccine.

 

[andyguitar]

When the reality probably is more like your very limited exposure was the equivalent of a vaccine.

Yes its naturally acquired immunity. Slightly bad news about covid is that it (probably) replicates quickly.

 

[Booble]

Interesting about the Chicken Pox. When I was a baby, my brother, age 5 -ish got 2 pox. Literally just 2. The doctor didn't believe it was chicken pox. Then my sister, age 2 got a fair amount pox. Then I, a six month old baby, was covered from head to toe. My mum said it was the grossest thing. They were up my butt, up my female private parts, covering my entire body.

 

[Gemini]

I was listening to a virologist and she said the severity of a covid infection often depends on how big the initial exposure to the virus was.

@ljimbo423 I find this interesting, too.

In a recent article Dr. Siddhartha Mukherjee notes since the start of COVID-19 the spread of virus across people has been measured, but now the spread of virus within people needs to be measured to answer three questions:

(1) What is the dose-response curve, i.e., risk of infection as people are exposed to higher "initial" doses of virus?

(2) What is the relationship between the initial dose and severity of disease, i.e., does higher dose result in graver disease?

(3) How does the virus behave in infected people, i.e., their peak viral load and its rise and fall that predict severity of illness and how infectious they are to others?

Answers could change the way patients are isolated, treated, and managed he feels. New Yorker, 4/6/20

 

[pattismith]

So if this happens to you you will test positive for antibodies meaning you were one of the lucky "asymptomatic" Which implies you were just better able to fight it off.

When the reality probably is more like your very limited exposure was the equivalent of a vaccine.

This is exactely what Pr Raoult is expecting in this article but admit it is yet not proven:

"In this context, the initial dose of the viral inoculum leading to infection may have a decisive impact on all subsequent events (Fig. 1B).

A small burden of SARS-CoV-2 should have a higher chance to stimulate a protective immune response than a high one, although additional factors like the fitness of the individual’s immune system and prior exposure to other in part cross-reactive CoVs might influence the outcome of the race between viral replication and T-lymphocyte responses as well.

Hence, it is possible, but remains to be demonstrated, that SARS-CoV-2 transmission from indolent or mildly symptomatic persons to naive individuals generally occurs at a relatively low viral load (lower than if the infection stems from severely affected patients), which then might have higher probabilities to induce immunity instead of severe and sometimes lethal infection (Fig. 1C). That said, current evidence suggests that the most solid predictors of disease severity after infection with SARS-COV-2 are the patient’s age and the concurrence of specific co-morbidities."

 

[andyguitar]

1) What is the dose-response curve, i.e., risk of infection as people are exposed to higher "initial" doses of virus?

(2) What is the relationship between the initial dose and severity of disease, i.e., does higher dose result in graver disease?

The higher the dose, the higher the chance that a virus will be able to infect a cell and go on to replicate. For a single virus to infect a cell it needs to first get through the protective layer of mucus. Then, as it cannot move independantly, it has to hitch a lift to get to a cell so it can infect it. So even if we have no immunity to it there are other ways in which it can be unsuccesful in causing an infection. The more virus particles = the more chances of a single virus getting through.

 

[antares4141]

@ljimbo423 I find this interesting, too.

In a recent article Dr. Siddhartha Mukherjee notes since the start of COVID-19 the spread of virus across people has been measured, but now the spread of virus within people needs to be measured to answer three questions:

(1) What is the dose-response curve, i.e., risk of infection as people are exposed to higher "initial" doses of virus?

(2) What is the relationship between the initial dose and severity of disease, i.e., does higher dose result in graver disease?

(3) How does the virus behave in infected people, i.e., their peak viral load and its rise and fall that predict severity of illness and how infectious they are to others?

Answers could change the way patients are isolated, treated, and managed he feels. New Yorker, 4/6/20

All this information I suspect would be expensive to obtain through animal models which may or may not translate well over to humans.

And of course unethical to perform on humans. IE we can't infect people in a controlled environment to obtain these types of data. At least I don't think we can.

Maybe there are clever ways around these limitations and I am unaware of them?

You can interview patients and collate data. But I suspect this type of data would be considered anecdotal and unreliable.

You could use data obtained from other non-leathal viruses where ethical considerations as far as purposely infecting subjects might be allowed.

With antibody test's we will be able to find out what percentage of the population was infected and recovered, their responses and what types of activities and circumstances like age comorbidity's etc. put them at risk.

This information will be very inexpensive relative to other types of studies, will be very useful and almost certainly will be done.

A lot of this data might not surface until after we have a vaccine though so how useful it becomes depends on the speed at which we find other ways to prevent and treat the condition.

 

[antares4141]

Interesting about the Chicken Pox. When I was a baby, my brother, age 5 -ish got 2 pox. Literally just 2. The doctor didn't believe it was chicken pox. Then my sister, age 2 got a fair amount pox. Then I, a six month old baby, was covered from head to toe. My mum said it was the grossest thing. They were up my butt, up my female private parts, covering my entire body.

Hard to read into this cause it was anecdote. But sounds like maybe you got a high degree of exposure and being a baby more susceptible. Which would be a double whammy, maybe triple if you were not fed breast milk which helps babies fight off infections when they are very young.

 

[andyguitar]

With antibody test's we will be able to find out what percentage of the population was infected and recovered, their responses and what types of activities and circumstances like age comorbidity's etc. put them at risk.

Yes this is the only way to get accurate figures. Over here in the uk there has been a total failure to obtain quality data. Deaths are being attributed to covid on the basis of a Doctors opinion not a lab test. This needs to stop.

 

[Booble]

Hard to read into this cause it was anecdote. But sounds like maybe you got a high degree of exposure and being a baby more susceptible. Which would be a double whammy, maybe triple if you were not fed breast milk which helps babies fight off infections when they are very young.

Wow - you nailed it. I was born in the early 1960s. No breast milk.

 

[antares4141]

Wow - you nailed it. I was born in the early 1960s. No breast milk.

Me too, actually mid 1959 but close. I wasn't fed breast milk either. Don't really know much about my exposure to chicken pox except I must have gotten it cause I just had an outbreak of shingles on my chin last november and it resulted in really bad tinnitus. My dr because I have CFS was hell bent on invalidating all my concerns and even though I showed him a picture of the outbreak at it's hight he said "it could have been horseflies". Only problem is there are no horseflies where I live in November.

 

[Hip]

I am just guessing this, but I suspect the minimum number of viral particles required to instigate an infection will also determine whether a respiratory virus is airborne or not.

Airborne means that the viral particles contained in tiny saliva droplets that float in the air are capable of causing infection if you breathe them in.

Only small saliva droplets of less than 10 μm in size are capable of floating in the air for some time; whereas large saliva droplets of greater than 20 μm in size are too heavy to float, and so fall to the ground within a meter of being ejected by the mouth. Ref: here.

I read that SARS-CoV-2 is not majorly spread by small airborne droplets, although it does happen; it is usually spread by the large droplets which have the 1 meter range (or by touch a surface where those large droplets have landed). Although there is some disagreement among experts on this.


But this, I am guessing, must mean that the quantity of SARS-CoV-2 viral particles contained in small airborne droplets is for the most part insufficient to cause an infection. Because if it was usually sufficient to cause an infection, then we would hear about lots of cases of airborne transmission.


Pathogens which are commonly transmitted by the airborne route include tuberculosis, measles and chickenpox (reference: see above link). So presumably small airborne saliva droplets will contain enough of the pathogen to cause an infection.

 

[antares4141]

Interesting. I would have thought the opposite. That airborne would be a major mode of transmission. And touching your face would be a lessor risk unless you threw all caution to the wind.

And you have a lot more control over hand face exposure if you are disciplined. Admittedly for me the hand face avoidance was extremely difficult and in the end I wasn't sure how well I did. But masks still allow some percentage of virus particles to pass. And there is nothing you can do about this.

For instance when somebody is talking they are mostly emitting the smaller droplets you mentioned. But they float for a long time. And so I would think it would be a numbers game. 50 small droplets are equal to one larger droplet or whatever the ratio might be.

But still if you spend enough time around an infected person and your breathing a bunch of the smaller droplets that would be like spending less time around somebody that coughed close enough to you to inhale some of the larger droplets

Or for instance 45 minutes in a grocery store where obviously you are going to have some level of small droplets floating in the air. With nothing but a mask for protection and the smaller droplets are going to have a better chance of getting through or around your mask.

In that situation best measure is order groceries online and have them brought out to your vehicle.

Total avoidance wherever possible, because counter measures are never going to be 100% effective.

I used to think I was more resistant to colds and flu cause I handn't had one as far as I can remember the first 10 years into my illness. But around 13 years in I had a pretty persistant cold and than again a couple of years later another persistant one.

No flu yet knock on wood.

But I, being 61 am pretty scared of covid19. Want to avoid it altogether if at all possible.